Hi there!I'm new to making AMVs and wondering if anyone knows which software are best for masking videos so that cool effects can be made.(e.g. two separate videos are overlaid so that only a character from one video appears on a different background)I've seen this done in Sony Vegas and actually want to know if there's any PPC compatible software out there that can do the same thing?

Assuming you mean PowerPC, old versions of After Effects (up to 7 I believe ran on PPCs) or Motion (up to FCS4.5). Finding copies that old, though, will be difficult. Neither Adobe nor Apple make those versions available anywhere, so your best option is probably ebay, sadly.

You're probably better off looking at getting a more recent system, though. An Intel system will give you access to a large number of useful tools, and Apple stopped manufacturing PPCs in 2006 and stopped supporting software for them last year.